July 23, 1998

A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.

Lava Flows Make Good Time Markers

Although the study of volcanoes is, in itself, fascinating and is more than
a full-time job, volcanologists also work closely with researchers in other
sciences. One of the things we contribute to the work of other scientists
is the ages of the lava flows around the island.

As described in a recent "Volcano Watch," the age of a lava flow can be
obtained by dating the charcoal found underneath it. The charcoal is all
that remains of plants burned by the advancing lava, so that the charcoal
and the lava flow have the same age. The ages of lava flows erupted since
the early 1800s are known from historical accounts.

Other scientists use those ages as starting points to figure out the rate
of growth of things that are found on those same lava flows. We think in a
similar way at home when we find unrecognizable items in the back of our
refrigerators: "Wow, this looks like a big oreo cookie, but it must be that
spamburger I bought last week! That green, furry mold with the pink flecks
must grow pretty fast."

The growth rates of plants, corals, and even beaches can be studied in the
same way. For example, most of the trees on the rim of Kilauea caldera
must be less than 200 years old because the soils they are growing in date
from about 1790. On or about that year, Kilauea erupted explosively and
apparently devastated most plant life in the vicinity. The trees near the
eastern portion of Saddle road are even shorter than both those on
Kilauea's rim and those farther away from the road because that roadway was
built on the 1855 and 1881 lava flows from Mauna Loa.

Dated lava flows can also be used as markers in Hawaiian archaeology.
Structures built on a lava flow must be younger than the lava flow.
Structures partially covered or surrounded by a lava flow must be older
than the lava flow. Using these rather obvious notions, petroglyphs found
on a recent Hawaiian flow can be used to question published ideas about
their interpretation.

Several books on Hawaiian petroglyphs point out that human figures are
generally found as either stick figures, figures with a triangular body, or
filled-in figures with muscular arms and legs. They then hypothesize that,
because the styles range from simple to complex, the simpler ones must be
older. The most recent petroglyphs should, according to this notion, be the
filled-in figures with musculature. However, both stick and triangular
human figures are found carved into the Hu`ehu`e lava flow erupted from
Hualalai in 1801! The geologic perspective would seem to support the idea
that the style of petroglyphs may be just that - a style.

Using these lava flow age markers, we have also raised questions about the
building of Hawaiian trails. For example, a beautiful, curbed trail extends
north from Kailua town along the coastline to the edge of the Hu`ehu`e
flow. The trail appears to continue as a slightly crooked path worn into
pahoehoe marked only by occasional bits of white coral. The curbed part of
the trail is thought to have been completed in the 1830s but ends at the
edge of the 1801 flow. Could the curbed trail be older than 1801?
Unfortunately, the place where the curbed trail meets the edge of the
Hu`ehu`e flow may have been destroyed during the construction of the Kona
airport. We cannot now tell whether 1801 lava flowed over the curbed trail.

Using flow features, we can sometimes guess about the environment into
which a particular lava flowed. Tree molds and wood remnants in those molds
can tell us what trees were growing there. Beach rocks frozen into the
surface of a lava flow can outline a former coastline. Using the Hu`ehu`e
flow example again, we know that both `ohi`a and wiliwili were common (only
wiliwili is present now) and that the pre-1801 coastline was as much as 3/4
mile inland of the present coastline.

We may only be scratching the surface of all the ways in which
volcanological information can be used in other scientific studies. Rest
assured that we will continue to seek them out.

Eruption and Earthquake Update

Kilauea's ongoing eruption paused briefly at the end of last week.
The pause started early on the morning of July 17 and ended about 48 hours
later on the 19th. During the pause, the lava tube leading from the vent
to the ocean drained, and blockages developed as the unsupported roof and
walls of the tube collapsed. When the eruption resumed, lava overflowed
from skylights in the blocked tube, forming several surface flows on Pulama
pali and the coastal plain. By evening on the 19th, lava had reoccupied
the tube all the way to the ocean, where it entered the water at the
Kamokuna site near the eastern boundary of Hawaii Volcanoes National Park.
One surface flow on the pali persisted through July 22.

The Big Island experienced one felt earthquake in the last week.
The magnitude 3.8 quake was located 28 km (17.5 mi) northeast of Kailua at
a depth of 28.6 km (17.7 mi).

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Contact:
hvowebmaster@usgs.govUpdated:
31 July 1998